The invention relates to a method for compensation control of offset voltages in baseband in a radio receiving circuit integrated in a circuit module which operates using a time-division multiple access method (TDMA) with reception timeslots. A sampling operational amplifier taps off a baseband output signal and an output of the sampling operation amplifier charges and discharges an externally fitted capacitor in dependence on a polarity of an offset voltage via a sampling switch of a sample and hold circuit. The sampling switch is switched on by an offset compensation enable (OCE) control signal before each reception timeslot in order to carry out an offset compensation process. A differential amplifier is used for comparing a voltage applied to the capacitor internally with a reference voltage resulting in a compensation current for counteracting the offset voltage.
The invention also relates to a circuit for carrying out the method.
In an integrated radio receiver module or radio transceiver module, a received signal (wanted signal) that is applied to the antenna in the radio frequency band is converted to an intermediate frequency band (IF) in the reception path, and is then further converted to baseband (BB) by a demodulator (mixer).
Offset voltages resulting from production tolerances can now occur in the reception path. The amplifier that is located in the reception path amplifies the offset voltages to the same extent as the wanted signal. If, for example, there is an offset voltage of 20 mV at the input of an amplifier, then the offset voltage at the output of the amplifier is 126 mV. The high offset voltage leads to limiting of the desired signal, since the offset voltage reduces the drive range of the amplifier and of all the subsequent amplifiers in the reception path.
In all high-frequency circuit parts (RF and IF bands), the offset voltage can be eliminated at suitable points by capacitive coupling. In baseband, in which the signal frequencies are about 100 kHz, capacitive coupling is, however, no longer possible since very large capacitances would have to be used for capacitive coupling in the integrated circuit module. However, such large capacitances require very long charging times when the circuit module is switched on and occupy a considerable area on the chip, or cannot be produced at all on an integrated circuit module.
It is known from European Patent Application EP 0 693 823 for the offset voltages in baseband to be compensated for by a sample and hold circuit. A differential baseband signal, which is obtained from an IF signal via a demodulator operating at a local oscillator frequency, and is filtered out after a buffer amplifier by a baseband filter circuit, is tapped off internally at the baseband output, which is connected to a downstream baseband processor, and is supplied to a sampling operational amplifier.
Depending on the polarity of the offset voltage, the operational amplifier charges and discharges a capacitor, which is fitted externally via an output, via a controlled sampling switch in the sample and hold circuit. The capacitor voltage is supplied to a differential amplifier, where it is compared with a reference voltage. Compensation currents that counteract the offset voltage are produced at the output of the differential amplifier. The offset compensation process is carried out before each reception timeslot in the radio receiver, which is operated using time division multiple access (TDMA). To this end, the sampling switch in the known circuit is controlled via an offset compensation enable (OCE) control signal, which is external to the module.
The OCE control signal requires its own pin and must be provided by the baseband processor, which is located in a separate chip. To produce the control signal, additional complexity is required in the baseband processor, and additional computer power is required. Such compensation for the offset voltage in baseband is provided, for example, in the Siemens PMB 2405 receiver chip, for portable GSM mobile radios.
An offset compensation circuit for a PCM telecommunications system is described in German Patent DE 30 41 417 C2. The circuit may be in the form of a component on an integrated circuit. In this case, the aim is to avoid discrete external components. A bipolar voltage generator and an integration circuit with switchable capacitors are provided for offset compensation.
It is accordingly an object of the invention to provide a method and a circuit for compensation control of offset voltages in a radio receiving circuit integrated in a circuit module which overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type, which make it possible for no specific pin to be required to carry out the offset voltage compensation in the radio receiver or transceiver chips, and which does not require any additional programming effort or extra computer power.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for compensation control of offset voltages in baseband in a radio receiving circuit integrated in a circuit module that operates using a time-division multiple access method (TDMA) with reception timeslots. The method includes using a sampling operational amplifier for tapping off a baseband output signal. An output of the sampling operation amplifier charges and discharges an externally fitted capacitor in dependence on a polarity of an offset voltage via a sampling switch of a sample and hold circuit. The sampling switch is switched on due to an offset compensation enable (OCE) control signal before each reception timeslot in order to carry out an offset compensation process. A differential amplifier compares a voltage applied to the capacitor internally with a reference voltage resulting in a compensation current for counteracting the offset voltage. The OCE control signal is supplied to the sampling switch within the circuit module and is generated in a sequencer functioning as a sequence controller provided in the circuit module. The sequencer is always started even before a start of an active TDMA reception timeslot and, for specific signals, emits a state sequence that is defined such that it is fixed in time, for a sequence control of the circuit module. The sequencer contains a running-down counter that, together with decoding logic at an output of the counter, emits the state sequence. The sequencer is started in dependence on three pulsed control signals which are input to the circuit module via a three-conductor bus and the sequencer starts when one of the three pulsed control signals has an edge, while the other two pulsed control signals are in a high state.
Thus, if a sequence controller, which is also referred to as a sequencer, is available on an integrated receiver or transceiver module, then, according to the invention, the process of offset voltage compensation can be carried out without any external OCE control signal having to be provided from the baseband processor.
A fully automatic offset voltage compensation sequence and the saving of a control pin on the integrated circuit module which contains the receiving circuit are thus achieved according to the invention in that control and programming signals, which are required in any case to set the desired states of the integrated circuit module are used to start the sequence controller (sequencer) which provides additional control signals, defined with respect to time, on the integrated circuit module (chip) without any additional computation power being required by the baseband processor contained in the radio.
The method and the circuit mentioned above for carrying out this method can be used particularly advantageously in a small transportable mobile radio (mobile telephone), which is used in a mobile radio system operating using time division multiple access, for example in a GSM or PCN/PCS mobile radio system.
In accordance with an added mode of the invention, there is the step of starting the sequencer using the three pulsed control signals when a programming of the circuit module ends, and one of the three pulsed control signals is an enable control signal available on the three-conductor bus.
In accordance with an additional mode of the invention, there is the step of starting the sequencer on one of a rising and falling edge of the enable control signal when the two other pulsed control signals are in a defined logic state.
In accordance with another mode of the invention, there are the steps of applying the OCE control signal to the sampling switch as a control signal to switch the sampling switch on for a fixed time period after the sequencer has started; and switching a state of the OCE control signal for turning off the sampling switch.
In accordance with a further mode of the invention, there is the step of setting a defined time period of an active state of the OCE control signal to be approximately 50 xcexcs.
In accordance with another additional mode of the invention, there is the step of stopping the sequencer once the running-down counter has run down and then restarting the sequencer anew due to the influence of the three pulsed control signals.
In accordance with a concomitant mode of the invention, there is the step of setting the defined logic state to be a high logic state.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit containing an integrated circuit module. The circuit module includes a sequencer with an output generating an offset compensation enable (OCE) control signal available at the output. A sampling switch having a control input is connected to the output of the sequencer and receives the OCE control signal. A three-conductor bus device is connected to the sequencer and receives three pulsed control signals passed on to the sequencer. The sequencer is configured such that the sequencer starts when one of the three pulsed control signals has an edge, while the other two of the three pulsed control signals are in a high state.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a circuit for compensation control of offset voltages in a radio receiving circuit integrated in a circuit module, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.